1. Field of Invention
The present invention relates to ophthalmic microsurgical instruments and, more particularly, to such surgical instrumentation which automatically controls internal ocular globe fluid pressure during ophthalmic surgical procedures and the like.
2. General Background
A large number of microsurgical procedures inside the eye are performed through "closed systems" which maintain the integrity and internal pressure of the ocular globe while microsurgical instruments are used to penetrate the eye through one or more small incisions (See FIG. 1). Exemplary functions performed by these instruments are:
fragmentation-the cutting and separation of ocular tissue, such as the lens in cataract surgery or fibrous and membrane-like growths inside the vitreous (e.g., vitrectomy, membraneoctomy);
emulsification-the mechanical digestion of tissue (usually the lens) by means of ultrasound in order to facilitate its removal through small incisions;
irrigation (infusion)-the introduction of a saline solution into the operating field by means of gravity or positive pressure; and
aspiration (suction)-the removal of fluid and/or entrained tissue fragments by means of vacuum.
The surgeon combines irrigation and aspiration to transport tissue fragments away from the operating field. He or she also uses these functions to maintain intraocular pressure during the surgical procedure. Control of pressure in irrigation and aspiration is extremely important. If aspiration suction is too strong (due to excessive vacuum), it may damage endothelial cells during anterior chamber surgery or may result in retinal detachment in vitrectomy procedures. Too high an irrigation pressure or excessive variations in the pressuree or flow rate of the irrigation fluid may traumatize ocular tissue.
Instruments for ophthalmic microsurgery made in accordance with prior art are based on the premise that the important parameters in the different surgical procedures are the static levels of intraocular pressure and aspiration vacuum. Static intraocular pressure is controlled by the height (hydrostatic head) of the infusion bottle that contains the saline solution used in ophthalmic surgery. Prior-art instruments provide for raising and lowering of the bottle at the surgeon's command using either manual or mechanical means. Likewise, aspiration vacuum can be controlled by the surgeon either presetting or continuously varying (via foot-pedal control) the pumping rate in the aspiration line (see for example Douvas: U.S. Pat. No. 4,168,707). In systems where measurement of intraocular pressure is attempted, a pressure sensor is typically placed (at some distance from the ocular globe as taught by Bittner U.S. Pat. No. 3,572,319) and manifested by current commercial instruments.
In February 1986 the inventors of the subject invention published the results of original research (Archives of Ophthalmology, Vol. 104, pp. 269-272) in which they demonstrated on the basis of theory as well as experimental data that the standard surgical maneuvers involved in common ophthalmic procedures (cataract surgery, vitrectomy) produce sudden, large changes in intraocular pressure. These pressure changes are due to perturbations in the rate of fluid flow into or out of the eye associated with enlargement or closing of incisions; the removal of tissue and vitreous humor; and the cutting action of surgical instruments inside the eye. Such sudden pressure changes include "spikes" with peak intensities as high as 160 mm Hg and rapid periodic fluctuations with frequencies as high as 300 cycles per minute. These dynamic changes in intraocular pressure cannot be controlled by manipulation of the infusion bottle height, nor can they be measured at remote locations, such as the console and the fluid line (where the pressure sensors are located in current, commercial instruments) due to rapid attenuation of the pressure disturbances, as they travel along the fluid conduit.
The research findings prompted the subject invention by the same inventors of the invention described in U.S. Pat. No. 4,548,205, which teaches the incorporation of pressure sensor/transducers into various types of infusion or mechanical cutting tips for use inside the eye, so as to provide signals for feedback control of irrigation or aspiration during ophthalmic procedures.